EDIT: This was only in reference to the Bluetooth retrofit, didn't mean to imply that the iPod adapter was a problem. It seems to run US$500-700 depending on the dealer and Mini...
Seems to fall in line with the reports on NAM with pricing that US dealers have been quoting (US$900-1200 for parts) but most of them seem to be saying they either won't do it or recommend against it. I haven't seen any reports of anyone actually having the retrofit done.
Sounds like the Bluetooth module is combined with other OBC bits, which makes for the expensive price.